The path of exhaust gas of an automobile is comprises an exhaust manifold, muffler, catalyst, flexible tube, center pipe, front pipe, and various other parts. When connecting these parts, fastening parts called “flanges” are often used. In parts of exhaust systems of automobiles, flange connections are being positively employed since flanges enable reduction of the processing steps and simultaneously enable reduction of the work space. Further, from the viewpoint of the noise due to vibration and securing rigidity, thick flanges of 5 mm thickness or more are often used.
Flanges have in the past been produced by press-forming, stamping, and otherwise processing ordinary steel sheet. However, ordinary steel is inferior in corrosion resistance, so after manufacture of the automobile, rust called “initial rust” occurs and the beautiful appearance is ruined. Therefore, instead of ordinary steel sheet, use of stainless steel sheet is being positively promoted as a material for flange.
Ferritic stainless steel sheet has less of a content of Ni and is lower in cost than austenitic stainless steel sheet, so mainly ferritic stainless steel sheet is often used for flanges, but there was a problem of inferior toughness. If steel sheet is low in toughness, the problem arises that plate fracture ends up occurring when running steel strip or uncoiling steel strip at a steel sheet production line. Further, in flange-making, cracks sometimes occur at the time of cutting, stamping, and other processing. Furthermore, when impacted in a low temperature environment in the winter, the problem arises that a flange ends up cracking and the automobile exhaust pipe ends up breaking. 5 mm or more thick ferritic stainless steel sheet is particularly low in toughness in some cases, so there was the issue that the reliability for use of the thick ferritic stainless steel sheet for the production of a flange was too low.
Therefore, to use the thick ferritic stainless steel sheet to produce a flange, the toughness of the steel sheet, in particular the toughness of the hot rolled steel sheet or hot rolled annealed and pickled steel sheet has to be improved. Several attempts have been made to solve the problems relating to the toughness of ferritic stainless steel sheet.
For example, PLTs 1 and 2 disclose manufacturing conditions for mass production of thickness 5 to 12 mm ferritic stainless steel hot rolled coil or hot rolled annealed coil. PLT 1 relates to Ti-containing ferritic stainless steel and shows the method of adjusting the hardness and Charpy impact value by making the coiling temperature 570° C. or more and dipping the coil in water. Further, PLT 2 relates to Nb-containing ferritic stainless steel and shows the method of adjusting the hardness and Charpy impact value by making the hot rolling finishing temperature 890° C. or more, coiling at 400° C. or less, and dipping the coil in water. The arts disclosed in PLTs 1 and 2 define the hot rolling conditions from the viewpoint of improvement of the toughness of the hot rolled plate or hot rolled annealed steel sheet, but control of the total length of a coil to the above conditions was difficult and the factors in metal structure for improvement of toughness were unknown.
Further, PLT 3 discloses ferritic stainless steel excellent in cold crack resistance obtained by making the length of subgrain boundaries with a small crystal misorientation of the ferrite phase a constant value or more. This ferritic stainless steel is obtained by the method of making the hot rolling finishing temperature 800 to 1000° C., making the coiling temperature over 650° C. to 800° C., and dipping in a water tank after coiling. Further, PLT 4 discloses ferritic stainless steel sheet excellent in toughness defining the ratio of the precipitates at the grain boundaries. The arts disclosed in PLTs 3 and 4 control the properties of the crystal grain boundaries and precipitates on the crystal grain boundaries so as to try to improve the toughness, but a level of toughness sufficient for a material for a flange has not necessarily been realized.